Unhappy son on notice (3)
I believe the answer is:
sad
'unhappy' is the definition.
(similar in meaning)
'son on notice' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'on' says to put letters next to each other.
'notice' becomes 'ad' (both can mean an advertisement).
's'+'ad'='SAD'
(Other definitions for sad that I've seen before include "Melancholy or pitiful" , "grey" , "Mournful - sorry" , "Downcast" , "Gloomy" .)